Airflow Academy

Do attic fans really work? This is a much-debated topic than I thought. There are HVACs and builders on one side that argues they are much needed, for every home, and then there are those (also in the construction/ air-conditioning industry) who argue they do worse than good. Who is correct? Here is a never-ending debate on this topic - (Please don't click it yet, check my next paragraph) https://www.energyvanguard.com/blog/75600/The-1-Reason-Power-Attic-Ventilators-Don-t-Help (Make sure you read the comments) What I am going to write here is the condensed version of all those arguments, so I think you are better off reading this article first and checking that link later. But let me first tell you how an attic fan works; How does an attic fan work? An attic exhaust fan is a powered attic ventilator which is used to push out the hot air in the attic during summer times and bring in the cooler air outside. The continuous replacement of the hot air with the cool air reduces the attic temperature. The attic fan is mounted on either the gable of the attic or on the roof. Be aware that the ones that are to be mounted on the roof cannot be installed on the gable, and vice versa. In both these locations, the fan is installed by cutting a whole and mounting the fan over it. While the fan is running it pushes the hot air out through these holes. When it creates a negative pressure in the attic the cool air from the outside will seep into the attic through the soffit and gable vents. A diagram showing the working of a roof mounted attic fan A good attic fan can reduce the attic temperature by 50 degrees on a hot day. But how effective are attic fans in cooling your house? While attic fans may be able to considerably reduce the temperature in your attic they may or may not help to reduce the temperature of the living areas. The effectiveness of attic fans in reducing your home temperature can be considered only on a case by case basis. One of the important considerations to deterine whether one needs an attic exhaust fan is the insulation of the ceiling. Is the insulation of your ceiling good? If so, then using an attic fan may not help much in reducing your room temperature. If the ceiling insulation is not good (that is if you are using an insulation less than R19) there are chances that the hot air from the attic can seep into your living areas. In this case using an attic fan can be effective. If the fan can reduce your room temperature by 10 degrees you can save around 20 to 30% in air conditioning costs. Also the HVAC ducts and equipments are installed in the attic of many houses. Unless these are 100% insulated they will absorb the heat in the attic which adds further load to the air conditioning. The use of an attic exhaust fan can reduce this energy loss to a great extent. In addition to reducing the attic temperature attic fans also helps to remove moisture from the attic thus preventing the build up of molds. What do naysayers say? The main arguments against the installation of powered attic fans (PAV) are; They do not help much in cooling the houseThe cooling achieved by attic fans is majorly due to the 'makeup air' ie, the cold air they suck from your living area.They are not cost-effective.More is saved by improving the insulation of the ceiling Let's check those points in detail; Myth 1. Attic fans do not help much in cooling the house The logic here is that the majority of the heat in the attic, and inside the house is due to radiation from the hot roof, and it is impossible to remove radiant heat through air circulation. Had this argument been correct then the air in the attic wouldn't be hot at all, only the roof deck and the other stuff like insulation, framing etc would be hot. Though air is a bad conductor it gets heated by convection ie, the hot air near the roof deck and the insulations will circulate spreading heat throughout the attic. So, does removing that hot air cool down the attic? Just imagine your car parked in the hot sun. It will be sizzling hot but when you open the doors/windows for a few minutes its interior will cool down. Your attic is just like that. That being said, this argument of radiating heat is not completely false either! It is NOT the hot air in the attic that heats up your living room. The ceiling absorbs radiant heat from the roof deck and transfers it down to your living areas. So removing the hot air in itself is not the solution. Doesn't that mean an attic fan is useless? Actually no. When there is circulation of colder air from the outside the radiant heat absorbed by the insulations and other materials there will be removed via convection. So this reduces the amount of heat transferred into the house. Myth 2. The cooling achieved by attic fans is by makeup air I can't say this is completely wrong. No ceiling is 100% leak-proof, so an attic fan will surely suck some of your cooled air. This happens when there is negative pressure in the attic. By providing proper roof vents, this can be avoided but not to 100%. HVI advises a minimum of 386 square inches of soffit ventilation for an attic fan of 700CFM. Myth 3. Attic fans are not cost-effective One of the many attractions of buying attic fans is that in time they will cover up their costs. Unfortunately, that may not be true for everyone. And to tell the truth, not everyone cares whether they are cost-effective; People collect a lot of stuff, and many keep them in their attic. In hot areas like Texas and Nevada, the attic temperature rises to 150 degrees. Just imagine you going there to collect things. So in most cases, it is a matter of convenience. And if you are using a solar attic fan it further reduces your electricity bills. Attic fans can be cost-effective in some houses and it may not be in some. Its performance and cost-effectiveness depends on a lot of things and varies from house to house so you kind of requires an 'attic audit'. Myth 4: More on air conditioning can be saved by improving the ceiling insulation This is actually true. If you can invest in bettering the heat insulations of your home that is more cost-effective than using an attic fan. But you will need an attic fan anyway if you are using the attic for storage. Do I really need an attic fan? As mentioned before whether you need an attic fan or not can be decided only on a case by case basis. There are cases when an attic fan is not the best solution but in most cases using attic fans is a good choice. An attic fan increases the air circulation through the roof, that's what it does. Any benefits projected by those who support attic fans are the result of this increased air circulation. If your ceiling has good heat insulation (R19 or above) I won't say an attic fan is the best idea economically. But you need an attic fan if the R rating is less than 14. If your home doesn't have air conditioning then using a whole house fan is far better than an attic fan. PS: Here is a list of some good attic fans if you are interested.